FIG. 29 is a perspective view of a conventional line filter. FIG. 30 is a cross sectional front view of the line filter. FIG. 31 is a chart of electric circuit used to describe transmission of a common mode noise in the line filter in operation. FIG. 32 is a chart of magnetic circuit used to describe flow of magnetic flux subject to a common mode noise in the line filter.
As shown in FIG. 29 and FIG. 30, a conventional line filter comprises a bobbin 72 having a through-hole 71, a first coil 73 and a second coil 74 wound around the bobbin 72, and a closed magnetic circuit core 75 which is inserted through the through-hole 71.
Referring to FIG. 31 and FIG. 32, in the means for eliminating common mode noise 76, magnetic flux AA and magnetic flux BB generated respectively from the first coil 73 and the second coil 74 proceed in a same direction in the magnetic core 75, in a manner they do not offset to each other.
Under the above described structure, when a common mode noise 76 is transmitted to an electric circuit as shown in FIG. 31, the magnetic flux AA and the magnetic flux BB proceed in the line filter along the directions as shown in FIG. 32.
Namely, the magnetic flux AA generated from the first coil 73 and the magnetic flux BB generated from the second coil 74 proceed in a same direction inside the magnetic core 75; joining together, in a manner not offsetting to each other.
Among the high frequency noise currents produced in an electric circuit through the commercial power supply are a common mode current and a differential mode current. The former current is called a common mode noise, the latter is called a normal mode noise.
A conventional line filter of the above structure is capable of eliminating the common mode noise 76, but it is unable to eliminate the normal mode noise.